Area-selective atomic layer deposition on 2D monolayer lateral superlattices

The advanced patterning process is the basis of integration technology to realize the development of next-generation high-speed, low-power consumption devices. Recently, area-selective atomic layer deposition (AS-ALD), which allows the direct deposition of target materials on the desired area using a deposition barrier, has emerged as an alternative patterning process. However, the AS-ALD process remains challenging to use for the improvement of patterning resolution and selectivity. In this study, we report a superlattice-based AS-ALD (SAS-ALD) process using a two-dimensional (2D) MoS2-MoSe2 lateral superlattice as a pre-defining template. We achieved a minimum half pitch size of a sub-10 nm scale for the resulting AS-ALD on the 2D superlattice template by controlling the duration time of chemical vapor deposition (CVD) precursors. SAS-ALD introduces a mechanism that enables selectivity through the adsorption and diffusion processes of ALD precursors, distinctly different from conventional AS-ALD method. This technique facilitates selective deposition even on small pattern sizes and is compatible with the use of highly reactive precursors like trimethyl aluminum. Moreover, it allows for the selective deposition of a variety of materials, including Al2O3, HfO2, Ru, Te, and Sb2Se3.


II. Supplementary Table
Energies (Erxn) for potential adsorption reactions on transition dichalcogenide (TMD) surface. In

Supplementary Note 1. Descriptions of CAS-ALD researches
Area-selective ALD (AS-ALD) is a method of selectively depositing a target material only on the desired region by creating a barrier which can give a difference in deposition rate 1-4 .For example, CH3 or CF3 tail group of a self-assembled monolayer (SAM) 1-3,5-9 , surface termination such as Si-H [10][11][12][13][14] , inherent selectivity in metal-insulator combination 4,15-18 , and non-dangling bond basal plane of 2D materials [19][20][21][22][23] can be used as a conventional AS-ALD (CAS-ALD) barrier.These barriers suppress chemisorption or reaction of specific precursors and prevent material deposition in a specific region.That is, by forming a barrier which is inert to a chemical reaction with the ALD precursors, deposition can only occur in the region without the barrier.
However, even if chemisorption of a specific precursor is prevented, material deposition can still occur on the barrier through physisorption due to van der Waals force between surface and precursors or penetration of precursors into barrier materials 2,3,5,20,24 .Many researches attempted to reduce the undesired deposition by physisorption through modification of process pressure or precursor types 2,3,5,13,14,24,25 , but fundamental elimination of such defects is challenging.In particular, it is well known that highly reactive and small-sized (low steric hinderance) precursors, such as trimethyl aluminum (TMA), can easily penetrate the barrier 5,13,24 , resulting in many defects.Furthermore, CAS-ALD needs pre-patterning process of substrate or barrier material by top-down lithography, which means resolution of CAS-ALD is limited by optical source 26,27 .Additionally, when the barrier materials, such as SAM molecule, are formed in a small area of a few nm scale, they cannot maintain their structures, leading to the pattern collapse and resolution degradation [28][29][30] .
strain, and Supplementary Figs.7c and 7f are TMD under tensile strain.Considering that the adsorption energy is stronger under compressive strain than tensile strain, the smaller isosurface in Supplementary Figs.7a and 7d indicates that the source of stronger adsorption in the compressed TMD region is not through chemical bonding character between the aluminum and the chalcogen atoms, but rather the van der Waals interaction between the precursor and the TMD.
(ii) Reaction between ALD precursor and TMD chalcogen vacancy sites As shown in HAADF-STEM image in Supplementary Fig. 1d, various types of defects are present, but do not present periodicity.As discussed in the main script, the initial nucleation presents such periodicity (See Fig. 3a and Supplementary Fig. 4e).Thus, it is quite evident that the defects are not the cause of SAS-ALD.Nonetheless, among the various defects, chalcogen vacancy sites (VS, VSe) are predominant in our superlattice, and we have also analyzed the reaction between these vacancy sites and ALD precursors.Supplementary Fig. 6b schematically illustrates the reaction where TMA removes one methyl group and bonds with DMA on a chalcogen vacancy site.In this reaction, both TMA-VS and TMA-VSe exhibit high reaction energy, 1.09 eV and 1.38 eV, respectively, indicating a relatively slow reaction (Supplementary Table 1).Furthermore, since the reaction energy is lower at VS than VSe, if the governing factor of nucleation is through these vacancies, the Al2O3 deposition should occur more easily on the MoS2 region, contrary to our case.Moreover, the reaction of H2O with the chalcogen sites, also show a reversed trend, with H2O-VS (1.21 eV) and H2O-VSe (1.28 eV), suggest that the reactions are more likely to occur on the MoS2 region.Hence, even in the case of chalcogen vacancy sites, the results contradict those observed in our SAS-ALD experiments.
The adsorption energy of TMA on chalcogen vacancy sites has also been calculated.On the MoS2 surface, TMA's adsorption energy is -0.66 eV, but on VS, it is -0.48 eV, indicating even less stable adsorption on vacancy site than non-defected MoS2 surface.Also, on the MoSe2 surface, adsorption energy is -0.67 eV, but on VSe, it is -0.48 eV.Therefore, TMA's adsorption is not stabilized on the chalcogen vacancy sites, again suggesting that vacancies don't have impact on the SAS-ALD mechanism.
(iii) Undesired deposition on multiple vacancies Considering the two situations mentioned above, it is clear that neither reactions nor chalcogen vacancies can explain our SAS-ALD.However, it has been confirmed that specific defects can lead to undesired deposition in SAS-ALD.Supplementary Fig. 8a shows SEM images for Al2O3 AS-ALD on a MoS2-MoSe2 lateral superlattice (i) immediately after CVD growth, (ii) 7 days later, and (iii) 14 days later.The superlattice was stored under ambient air condition.As seen in the SEM images, it is evident that the longer the superlattice is exposed in the air, the more undesired Al2O3 deposition occurs on the MoS2 region.
The cause of this selectivity loss was analyzed through TEM measurements.Supplementary Figs.8b-d are HAADF-STEM images of the superlattice that was not degraded in air condition.
In the interface area (Supplementary Fig. 8c) and in each MoS2 and MoSe2 region (Supplementary Fig. 8d), similar to Supplementary Fig. 1d, there are slight chalcogen vacancies, but no significant defects are found.However, in the superlattice that degraded over 14 days under air conditions, many large multiple vacancies were observed at the interface and in each MoS2 and MoSe2 region (red circles in Supplementary Figs.8e-g).These large multiple vacancies, resulting from the degradation of lateral superlattice, occur in both MoS2 and MoSe2 areas, are found to cause the undesired deposition in SAS-ALD.Importantly, as the defect size is a few nanometer scales, the SiO2 substrate underneath is exposed, making it highly likely for TMA to reaction in these regions.However, it is evident that these large defects, present only in the degraded superlattice, are significantly different from our SAS-ALD process described in the main script.

Fig. 1
Fig. 1 Additional characterizations of monolayer MoS2-MoSe2 lateral superlattice Fig. 2 Various materials AS-ALD on the monolayer MoS2-MoSe2 lateral superlattice Fig. 3 Additional characterizations of Al2O3 SAS-ALD Fig. 4 Al2O3 SAS-ALD on MoS2-MoSe2 lateral superlattice with large MoSe2 width Fig. 5 Additional MD simulation mapping Fig. 6 Schematic of dissociative adsorption of TMA on TMD Fig. 7 Charge density difference plots of TMA on TMD surfaces for chemisorption selectivity Fig. 8 Selectivity loss in SAS-ALD by multiple vacancy defects Fig. 9 TMA adsorption energy under separate x and y lattice straining Fig. 10 TMA adsorption on concave surface of TMD Fig. 11 Adsorption selectivity of H2O in SAS-ALD Fig. 12 Adsorption energy of ligand in ALD precursors for Al2O3, Sb2Se3, HfO2, and Te Fig. 13 Adsorption energy of other ALD precursors Fig. 14 Gibbs free energy for adsorption of TMA on 2D surface and the diffusion barriersFig.15 Al2O3 ALD on single MoS2 and MoSe2 surface Fig. 16 Diffusion and desorption rate varying with temperature Fig. 17TMA equilibrium coverage obtained by kMC Fig. 17TMA equilibrium coverage obtained by kMC Fig. 18 Heatmap of TMA collisions with varying temperature Fig. 19 Al2O3 SAS-ALD on WS2-WSe2 lateral superlattice Fig. 20 Interface quality and optical property of selectively deposited aluminum oxide Fig. 21 Electrical characterization of MoSe2 nanoribbon FET (NRFET) the reaction formula, asterisk (*) indicates the adsorption site.The type of adsorption site is denoted in the reaction site(s) column, where Se, VSe, S, and VS represent the Se top, Se vacancy, S top, and S vacancy site in MoSe2 and MoS2 surfaces, respectively.For dissociative reactions, the sites are numbered according to their appearance in the right-hand side of the reaction formula.Adsorption on -3 % lattice strained MoSe2 surface is indicated in parentheses.The precursors are trimethyl aluminum (TMA), dimethyl aluminum (DMA), and H2O.